Fluidized bed combustion system and method having multiple furnace and recycle sections

ABSTRACT

A fluidized bed combustion system and method in which a plurality of enclosures, each having a furnace section and integral recycle section are joined using common walls. Openings are provided in lower portions of the common walls for permitting the fluidized bed material to flow between adjacent or opposing furnace sections and between adjacent recycle sections. Openings are also provided in upper portions of the common walls for equalizing the gas pressure between the adjacent or opposing furnace sections.

BACKGROUND OF THE INVENTION

This invention relates to a combustion system and method, and, moreparticularly, to such a system and method in which a plurality ofadjacent and opposing enclosures including furnace sections and recyclesections are provided for receiving fluidized beds.

Fluidized bed combustion systems are well known and include a furnacesection in which air is passed through a bed of particulate material,including a fossil fuel, such as coal, and a sorbent for the oxides ofsulfur generated as a result of combustion of the coal, to fluidize thebed and to promote the combustion of the fuel at a relatively lowtemperature. These types of combustion systems are often used in steamgenerators in which water is passed in a heat exchange relationship tothe fluidized bed to generate steam and permit high combustionefficiency and fuel flexibility, high sulfur adsorption and low nitrogenoxides emissions.

A typical fluidized bed utilized in the furnace section of these typesystems is commonly referred to as a "bubbling" fluidized bed in whichthe bed of particulate material has a relatively high density and a welldefined, or discrete, upper surface. Other types of systems utilize a"circulating" fluidized bed in which the fluidized bed density is belowthat of a typical bubbling fluidized bed, the fluidizing air velocity isequal to or greater than that of a bubbling bed, and the flue gasespassing through the bed entrain a substantial amount of the fineparticulate solids to the extent that they are substantially saturatedtherewith.

Circulating fluidized beds are characterized by relatively high internaland external solids recycling which makes them insensitive to fuel heatrelease patterns, thus minimizing temperature variations and, therefore,stabilizing the sulfur emissions at a low level. The external solidsrecycling is achieved by disposing a cyclone separator at the furnacesection outlet to receive the flue gases, and the solids entrainedthereby, from the fluidized bed. The solids are separated from the fluegases in the separator and the flue gases are passed to a heat recoveryarea while the solids are recycled back to the furnace. This recyclingimproves the efficiency of the separator, and the resulting increase inthe efficient use of sulfur adsorbent and fuel residence time reducesthe adsorbent and fuel consumption. U.S. Pat. Nos. 5,040,492 and5,054,436, assigned to the same assignee as the present application,disclose systems in which the separated solids are recycled back to thefurnace.

U.S. Pat. Nos. 4,609,623 and 4,809,625, assigned to the same assignee asthe present application, disclose a fluidized bed reactor in which adense, or bubbling, bed is maintained in the lower portion of thefurnace, while the bed is otherwise operated as a circulating bed. Thishybrid arrangement results in several advantages not the leastsignificant of which is the ability to utilize fuel and adsorbent over arelatively large particle size range.

In designing fluidized bed combustion systems of the above types,increases in furnace capacity from a given design are usually achievedby increasing the height of the furnace walls. However, this isexpensive and there are certain limits to the height of the walls. Ithas therefore been suggested that the size of the furnace, and thereforeits capacity, be increased by increasing the size of the furnace in"plan view" i.e., increasing the width and/or the depth of the furnace.However, this usually requires a common wall, or the like, to be placedin the furnace section to divide the area into two or more fluidizedbeds which requires separate operating controls, etc. which isexpensive. Also, the common wall is subjected to lateral loading,especially when the multiple beds operate differently or if one bed isrendered inoperable due to equipment failure. This lateral loading cancause damage to the wall and attendant reduction i operation andefficiency.

Increases in furnace capacity also lead to the use of larger cycloneseparators which permit increasing amounts of fine, unburnt fuelparticles to escape with the separated flue gases. This escape ofunburnt fuel particles reduces fuel efficiency, thereby increasing fuelconsumption.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide afluidized bed combustion system and method which has an increasedcapacity without any increase in furnace wall height.

It is a further object of the present invention to provide a fluidizedbed combustion system and method of the above type in which two or moreseparate enclosures having furnace sections are provided.

It is a further object of the present invention to provide a fluidizedbed combustion system and method of the above type in which two or moreenclosures having furnace sections and integral recycle sections areprovided.

It is a further object of the present invention to provide a fluidizedbed combustion system and method of the above type in which theenclosures are joined by common walls.

It is a still further object of the present invention to provide afluidized bed combustion system and method of the above type in whichseparate furnace sections may be operated without the need for separatecontrols.

It is a still further object of the present invention to provide afluidized bed combustion system and method of the above type in whichseparate integral recycle sections may be operated without the need forseparate controls.

It is a still further object of the present invention to provide afluidized bed combustion system and method of the above type in whichthe common walls are vented to equalize the pressure across the walls,to minimize or eliminate lateral loading and to enable the fluidizedbeds in each furnace section to maintain substantially the same height.

It is a still further object of the present invention to provide afluidized bed combustion system and method of the above type in whichthe common walls are vented to enable the fluidized beds in the integralrecycle sections to maintain substantially the same height.

It is a still further object of the present invention to provide afluidized bed combustion system and method of the above type in whichfuel efficiencies are increased by reducing losses of fine, unburnt fuelparticles.

Toward the fulfillment of these and other objects, according to thesystem and method of the present invention a plurality of enclosures,each having a furnace section and a recycle section, are joined usingcommon walls A fluidized bed of particulate material is formed in eachfurnace section and recycle section and the flue gases in each furnacesection entrain portions of the particulate material and rise upwardlyin each furnace section before discharging to a cyclone separator or thelike. Upper and lower portions of the common walls are vented toequalize pressure across the walls and to maintain substantiallyidentical dense bed heights in the furnace sections and substantiallyidentical dense bed heights in the recycle sections.

BRIEF DESCRIPTION OF THE DRAWINGS

The above brief description, as well as further objects, features andadvantages of the present invention will be more fully appreciated byreference to the following detailed description of the presentlypreferred but nonetheless illustrative embodiments in accordance withthe present invention when taken in conjunction with the accompanyingdrawings wherein:

FIG. 1 is a schematic representation depicting the system of the presentinvention;

FIG. 2 is an enlarged cross-sectional view taken along the line 2--2 ofFIG. 1;

FIG. 3 is a reduced cross sectional view taken along the line 3--3 ofFIG. 2;

FIG. 4 is a cross-sectional view taken along the line 4--4 of FIG. 2;and

FIGS. 5 and 6 are views similar to FIG. 2 but depicting alternateembodiments of the system of the present invention.

FIG. 7 is an enlarged cross-sectional view taken along the line 7--7 ofFIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2 of the drawings, the fluidized bed combustionsystem of the present invention is referred to in general by thereference numeral 10. The system 10 includes two upright water-cooledenclosures 12a and 12b which are substantially identical. For theconvenience of presentation, only enclosure 12a will be described indetail.

The enclosure 12a has a front wall 14a, a rear wall 16a and twosidewalls, one of which is referred to by the reference numeral 17 andthe other of which is formed by a common wall extending between theenclosures 12a and 12b, which will be discussed in detail later. Theupper portion of the enclosure 12a is closed by a roof 18a and the lowerportion includes a floor 20a. A distribution plate or grate 22a extendsacross a lower portion of the enclosure 12a. The plate 22a is spacedfrom the floor 20a to define a plenum area between the floor 20a and theplate 22a which is adapted to receive an oxygen-containing gas such asair from an external source (not shown).

A partition 24a is disposed in the enclosure 12a and extends between theside walls, including side wall 17a. The partition 24a includes a lower,substantially vertical portion 24a' which extends upwardly from thefloor 20a, through the distribution plate 22a, and into the enclosure12a. The partition portion 24a' is disposed parallel to the front andrear walls 14a and 16a. The partition 24a also includes an upper portion24a" which angles upwardly and rearwardly from the lower portion 24a' ofthe partition to the rear wall 16a. The partition 24a thereby dividesthe plenum area into plenum chambers 26a and 28a and further divides theenclosure 12a into a furnace section 30a disposed above plenum chamber26a and a recycle section 32a disposed above plenum chamber 26a. Atleast one opening 34a is provided in a lower portion of the verticalpartition portion 24a' for reasons to be described. A plurality of airdistributor nozzles 36a are mounted in corresponding openings formed inthe portion of the plate 22a extending under the furnace section 30a forpassing air through the plate 22a, for reasons to be described.

A feeder system 38a is provided adjacent the front wall 14a forintroducing particulate material into the furnace section 30a. Theparticulate material includes fuel and may also include other componentsincluding an adsorbent, such as limestone.

The particulate material is fluidized in the furnace section 30a by theair from the plenum 26a as it passes upwardly through the plate 22a.This air promotes the combustion of the fuel, generating combustiongases which combine with the air to form flue gases which rise in thefurnace section 30a by convection and which entrain a portion of theparticulate material as will be described.

A cyclone separator 40a extends adjacent the enclosure 12a. As shown inFIG. 1, a duct 42a extends from an outlet opening provided in the rearwall 16a of the enclosure 12a to an inlet opening provided through thewall of the separator 40a. The separator 40a receives flue gases andentrained particulate material from the furnace section 30a in a mannerto be described and operates in a conventional manner to disengage theparticulate material from the flue gases. The separated flue gases inthe separator, which are substantially free of solids, pass, via a duct44a located immediately above the separator, into a heat recoverysection 48, via an inlet provided through a wall thereof.

The heat recovery section 48 includes a plurality of heat exchangesurfaces 50 which may serve as heaters, reheaters, superheaters, andeconomizers, all of which are formed by a plurality of heat exchangetubes extending in the path of the gases as they pass through the heatrecovery section 48. After passing across the heat exchange surfaces 50,the gases exit the heat recovery section 48 through an outlet 52. It ispreferred that a number of separators associated with additionalenclosures be connected to a single heat recovery section 48. It isunderstood that a number of different embodiments of heat recoverysections may be used. For a more detailed discussion of a preferredembodiment of a heat recovery section, see U.S. Pat. Nos. 5,040,492 and5,054,436, both assigned to the assignee of the present invention. Thedisclosure of these references is incorporated herein by reference.

As shown in FIG. 1, the lower portion of the separator 40a is conicallyshaped and is connected to a dip leg 54a which, in turn, is connected toa J-valve 56a. A conduit 58a connects the outlet of the J valve 56a tothe recycle section 32a to transfer the separated particulate materialfrom the separator 40a to the recycle section 32a. The J valve 56afunctions in a conventional manner to prevent back-flow of solids fromthe furnace section 30a and the recycle section 32a to the separator40a. It is understood that a substantially identical separator, dip leg,J-valve and inlet conduit are associated with each enclosure and allfunction in a substantially identical fashion. It is also understoodthat other types of separators may be used, and the separators may passthe separated particulate material to the recycle sections in anyconventional manner.

As shown in FIG. 2, the enclosures 12a and 12b are disposed adjacent oneanother and share a common wall 60 which extends from front walls 14aand 14b to rear walls 16a and 16b and which extends from the enclosurefloors 20a and 20b to the enclosure roofs, including roof 18a. A furnacesection 30b is formed in the enclosure 12b and is disposed adjacent thefurnace section 30a, and a recycle section 32b located adjacent thefurnace section 30b and is disposed adjacent the recycle section 32a.

As shown in FIG. 3, a plurality of openings 62 are provided in the upperportion of the common wall 60 and a plurality of openings 64 and 66 areprovided in the lower portion of the common wall 60, for reasons to bedescribed.

FIGS. 2 and 4 depict the adjacent recycle sections 32a and 32b ingreater detail. For convenience of presentation, the recycle section 32awill be described in detail, it being understood that the descriptionalso applies to the recycle section 32b, as well as to additionalrecycle sections such as those in FIGS. 5 and 6.

A partition 68a is disposed in the recycle section 32a and extendsbetween the side wall 17a and the common wall 60 and parallel to thevertical partition portion 24a'. The partition 68a also extends from thedistribution plate 22a to the angled portion 24a" of the partition 24ato define a channel 70a between the partitions 24a and 68a. A pluralityof openings are provided in an upper portion of the partition 68a forreasons to be described.

The front wall 14a, the rear wall 16a, the sidewalls, the roof 18a, thepartitions 24a and 68a, and the walls of the separator 40a and heatrecovery section 48 are all formed by a plurality ofvertically-extending, spaced, parallel tubes 72 with adjacent tubesbeing connected by continuous fins 74 along their lengths to formairtight structures. As shown schematically in FIG. 1, a portion of thetubes 72 forming the rear wall 16a are bent out of the plane of thelatter wall, towards the partition section 24a" to form a partition 76a,and back to the rear wall 16a to form a partition 78a. The partitions76a and 78a thus help support the partition section 24a".

A pair of vertically-spaced secondary air inlets 80a and 82a registerwith openings in the rear wall 16a for introducing a secondary,oxygen-containing gas such as air into the enclosure 12a at two levels,one between the points of intersection of the partitions 76a and 78awith the rear wall 16a and another above the point of intersection ofthe partition 78a with the rear wall 16a. Although not clear from thedrawings, it is understood that the tubes 72 forming the partition 76ahave no fins so that secondary air from the inlet 80a can passtherethrough, while the tubes 72 forming the partition 78a are finned toprevent the passage of air therethrough and thus form a roof for therecycle section 32a.

As shown in FIG. 1, four rows of nozzles 84a extend through thepartition portion 24a", with two rows located above the partition 78aand two rows located below the partition 78a. As a result, secondary airfrom the inlet 80a is directed through the lower two rows of nozzles84a, and secondary air from the inlet 82a is directed through the uppertwo rows of nozzles 84a.

As best shown in FIGS. 2 and 4, partitions 88a and 90a are disposedwithin the recycle section 32a and extend between the rear wall 16a andthe partition 68a, substantially parallel to the side wall 17a and thecommon wall 60. The partitions 88a and 90a extend upwardly from thedistribution plate 22a to a desired height within the recycle section32a. Referring to FIG. 4, the partitions 88a and 90a divide the lowerportion of the recycle section 32a into three compartments 92a, 94a and96a. As shown in FIG. 2, the inlet conduit 58a registers with an openingin the rear wall 16a communicating with the compartment 94a.

Within the recycle section 32a, a plurality of rows of nozzles 98aextend through the perforations in the plate 22a above plenum chamber28a. Each nozzle 98a consists of a central portion extending through theperforation and a horizontal discharge portion registering with thevertical portion. The nozzles 98a in the compartments 92a and 96a aredisposed in parallel rows with their discharge portions facing away fromthe compartment 94a. Two parallel rows of nozzles 98a are provided inthe compartment 94a with their discharge portions facing towards thepartitions 88a and 90a, respectively. A single row of nozzles 100a isalso located in the compartment 94a and extends between the two rows ofnozzles 98a. The nozzles 100a are taller than the nozzles 98a forreasons to be explained. A manifold 102a is located in the plenum 28aand is connected to the nozzles 100a for supplying air to the nozzles100a independently of the flow of air from the plenum 28a, through theplate 22 and to the nozzles 98a.

As shown in FIG. 4, a bank of heat exchange tubes 104a are disposed ineach of the compartments 92a and 96a. The tubes 104a are bent into aserpentine pattern and extend between headers for circulating fluidthrough the tubes 104a in a conventional manner.

Three horizontally-spaced, elongated slots or openings 106a, 108a and110a (FIG. 4) are provided through a portion of the partition 68adefining the compartments 92a, 94a and 96a, respectively. The opening108a extends at an elevation higher than the openings 106a and 110a forreasons to be described. The openings are shown schematically in FIG. 4for the convenience of presentation, it being understood that theyactually are formed by cutting away the fins 74, or bending the tubes 72out of the plane of the partition 68a. A plurality of openings 112a and114a are formed in the lower portions of the partitions 88a and 90a,respectively, to communicate the chambers 92a and 96a with the chamber94a. As shown in FIG. 2, the common wall 60 extends to the rear wall 16ato separate the recycle sections 32a and 32b and a plurality of openings66 are provided in the extended portion of the common wall 60, forreasons to be described.

It is understood that the particular design of recycle sections 32a and32b is shown as an example only and that a number of differentembodiments of recycle sections may be used. For example, U.S. Pat. Nos.5,054,436 and 5,040,492, both assigned to the assignee of the presentapplication, disclose a number of different recycle sectionconfigurations that may be employed with the present invention. Thedisclosure of these references is incorporated herein by reference.

It is understood that the above description of the disclosure 12a isequally applicable to the enclosure 12b and identical structure in thelatter embodiment is indicated by the same reference numerals but with a"b" suffix. Therefore, the enclosure 12b will not be described indetail.

A steam drum 116 (FIG. 1) is located above the system 10 and, althoughnot shown in the drawings, it is understood that a plurality of headersare disposed at the ends of the various water-tube walls describedabove. As shown in general by the reference numeral 118, a plurality ofdowncomers, pipes, etc. are utilized to establish a flow circuit forcirculating a cooling fluid such as water or steam or a water and steammixture through these headers, the steam drum 116, and the various tubedwalls, partitions, and heat exchange surfaces, with connecting feeders,risers, and headers being provided as necessary. Thus, water is passedin a predetermined sequence through this flow circuitry to convert thewater to steam and to heat the steam by the heat generated by combustionof the particulate fuel material.

For ease of presentation, the operation of the present system will bedescribed with reference to enclosure 12a. In operation, particulatematerial including fuel and sorbent material are introduced into thefurnace section 30a through the feeder system 38a. Alternately, sorbentmay also be introduced independently through openings formed through oneor more of the enclosure walls. Air from an external source isintroduced at a sufficient pressure into the plenum 26a extending belowthe furnace section 30a, and the air passes through the nozzles 36adisposed in the furnace section 30a at a sufficient quantity andvelocity to fluidize the particulate material in the furnace section30a. Each nozzle 36a is adjusted so that the velocity of the airdischarged therefrom increases from right-to-left as viewed in FIG. 1,i.e., the nozzles 36a closest to the front wall 14a discharge air at arelatively high velocity while the nozzles 36a closest to the partition24a discharge air at a relatively low velocity.

A liftoff burner (not shown), or the like, is provided to ignite thefuel material, and thereafter the fuel material is self-combusted by theheat in the furnace section 30a. Combustion of the fuel materialgenerates combustion gases which mix with the air introduced through theplate 22a, which mixture is hereinafter referred to as flue gases. Theflue gases pass upwardly through the furnace section 30a and entrain, orelutriate, a portion of the particulate material.

The quantity of particulate material introduced into the furnace section30a and the quantity of air introduced, via the air plenum 26a, throughthe nozzles 36a and into the interior of the furnace section 30a isestablished in accordance with the size of the particulate material sothat a dense bed is formed in the lower portions of the furnace section30a and a circulating fluidized bed is formed in the upper portionsthereof, i.e. the particulate material is fluidized to an extent thatsubstantial entrainment or elutriation thereof is achieved. Operated inthe above manner, the density of the particulate material is relativelyhigh in the lower portion of the furnace section 30a, decreases withheight throughout the length of the furnace section 30a and issubstantially constant and relatively low in the upper portions of thefurnace section 30a. Since the operation in the enclosure 12b isidentical to that in the enclosure 12a, the former will not be describedin detail.

As best shown in FIGS. 3 and 4, the openings 64 in the lower portion ofthe common wall 60 are sized to permit adequate flow of the particulatematerial between the furnace sections 30a and 30b so that the respectiveheights of the solids in the furnace sections 30a and 30b section aresubstantially the same.

Referring again to FIG. 1, the flue gases passing into the upper portionof the furnace section 30a are substantially saturated with theparticulate material and pass, via the outlet opening in the upperportion of the rear wall 16a, into the cyclone separator 40a. Theopenings 62 (FIG. 3) in the upper portion of the common wall 60 equalizethe gas pressure in the furnace sections 30a and 30b and thus eliminateany pressure drop across the common wall 60.

In the separator 40a, the particulate material is separated from theflue gases, and cleaned flue gases pass to the heat recovery section 48for passage across the heat exchange surfaces 50. The separatedparticulate material passes from the separator 40a, through a dipleg54a, J-valve 56a, and conduit 58a, and into the recycle section 32a asdescribed above.

With reference to FIGS. 2 and 4, the separated solids from the conduit58a enter the compartment 94a of the recycle section 32a. Assumingnormal operation, the plenum chambers 26a and 28a selectively distributethe air through the nozzles 36a and 98a, respectively, to the furnacesection 30a and the recycle section 32a. Each nozzle 36a and 98a is ofconventional design and, as such, includes a control device to enablethe velocity of the air passing therethrough to be controlled. Duringsuch normal operation, fluidizing air is introduced, via the plenum 28a,to the nozzles 98a in the compartments 92a, 94a and 96a of the recyclesection 32a, while the air flow to the manifold 102a, and therefore tothe nozzles 100a, is turned off. Since the two rows of nozzles 98a inthe compartment 94a are directed towards the partitions 88a and 90a, theparticulate material passes from the compartment 94 a into thecompartments 92a and 96a.

The particulate material mixes and builds up in the compartments 92a and96a and thus gives up heat to the water/steam in the tubes 104a in thosecompartments. The cooled particulate material then passes through theopenings 106a and 110a in the partition 68a through the channel 70a(FIG. 1), through the openings 34a in the partition 24a, and back intothe furnace section 30a.

As shown in FIGS. 3 and 4, the openings 66 in the common wall 60 aresized to permit adequate flow of particulate material between thecompartments 96a and 92b of the recycle sections 32a and 32b,respectively, so that the respective heights of the particulate materialin the compartments 92a, 96a, 92b are maintained substantially the same.

Since, during the above operation there is no air introduced into thenozzles 100a in the compartment 94a, very little, if any, flow ofparticulate material occurs through the compartment 94a and opening108a. During initial start up and low load conditions, the fluidizingair flow to the plenum 28a is turned off and the air flow to themanifold 102a, and therefore to the nozzles 100a, is turned on. As aresult, the volume of particulate material in the compartments 92a and96a slump and therefore seal these compartments from further flow. Thus,the separated particulate material from the conduit 58a passes directlythrough the compartment 94a and, after building up to the level of theopening 108a, passes through the opening 108a, through the channel 70a,through the openings 34a in the partition 24a, and back into the furnacesection 30a. Since the compartment 94a does not contain heat exchangetubes 104a, start up and low load operation can be achieved withoutexposing the banks of tubes 104a to the hot recirculating particulatematerial.

If desired, secondary air may be introduced into the enclosure 12a viainlets 80a and 82a, the secondary air from inlet 80a passing through thespaces in the partition 76a and through the recycle section 32a beforeexiting through the lower two rows of nozzles 84a leading into thefurnace section 30a. The secondary air from the inlet 82a is preventedfrom passing into the recycle section 32a by the partition 78a andtherefore passes through the upper two rows of nozzles 84a leading intothe furnace section 30a. The fluidizing air that is introduced into therecycle section 32a is controlled to entrain fine fuel particles in therecycle section. In this manner, fine fuel particles of approximately 1to 10 micrometers in diameter are exposed to the secondary air from thesecondary air inlet 80a and pass with the secondary air through thenozzles 84a into the furnace section 30a. The high oxygen content in thelatter air promotes the combustion of these entrained fine fuelparticles as they pass from the recycle section 32a, through the lowertwo rows of nozzles 84a and into the furnace section 30a.

Feed water is introduced into the flow circuit described above and iscirculated therethrough in a predetermined sequence to convert the feedwater to steam and to reheat and superheat the steam.

Also, drain pipes (not shown) may be provided for the furnace section30a and recycle section 32a and for each furnace section and recyclesection as desired for discharging spent particulate material, in aconventional manner.

The system and method of the present invention have several advantages.For example, in the embodiment of FIGS. 1-4, the use of two adjacentenclosures sections 12a and 12b sharing a common wall 60 enables thesize of the system 10, and therefore the load capacity, to be increasedwithout increasing the height of the system. Moreover, the provision ofopenings 64 and 66 provided in the lower portion of the common wall 60equalizes the heights of the respective dense beds in the furnacesections 30a and 30b and recycle sections 32a and 32b, thus correctingfor imbalances in the fuel feed from the feeder systems 38a and 38b, orthe like. Further, since the adjacent enclosures 12a and 12b aresubstantially the same, a single control scheme can be utilized whichcontrols the operation in both enclosures. Moreover, the provision ofthe openings 62 in the upper portion of common wall 60 enables therespective gas pressures in the furnace sections 30a and 30b to beequalized, thus minimizing or eliminating any lateral loading across thecommon wall 60 and possible damage. Also, the openings 62 enable apredetermined gas pressure drop to be set across the furnace sections30a and 30b and enable the entrainment and circulation to besubstantially the same in each furnace section 30a and 30b. Also, theprovision of the openings 62 enables substantially the same combustionenvironments to be established above the dense bed in both furnacesections 30a and 30b.

Although the embodiment described above utilizes a single common wall 60shared between two substantially identical enclosures 12a and 12b, it isunderstood that multiple common walls can be used in a similar manner tojoin additional enclosures. As examples of this, FIGS. 5 and 6 depictalternate embodiments of the present invention in which adjacent andopposing enclosures are joined by common walls.

According to the embodiment of FIG. 5, two more enclosures 12c and 12d,which are substantially similar to enclosures 12a and 12b, are joinedwith enclosures 12a and 12b. Enclosure 12c is disposed oppositeenclosure 12a and adjacent enclosure 12d, and enclosure 12d is disposedopposite enclosure 12b. Enclosure 12a and enclosure 12c share a commonwall 120, and enclosure 12b and enclosure 12d share a substantiallyidentical common wall 122. As shown in FIG. 7, common wall 122 (andcommon wall 120) has a plurality of openings 124 in an upper portion forpermitting flue gases to pass between the furnace sections joinedthereby. The openings 124 equalize gas pressure in the joined furnacesections and thus eliminate any pressure drop across the common wall122. The common wall 122 (and common wall 120) also has a plurality ofopenings 126 in a lower portion for permitting particulate material toflow between the furnace sections joined thereby. The openings 126 aresized to permit adequate flow of particulate material between thefurnace sections joined thereby so that the respective heights of thedense beds in the furnace sections are substantially the same. Thecommon wall 128 between enclosure 12c and enclosure 12d is substantiallyidentical to common wall 60. Separators 40a, 40b, 40c, and 40d areassociated with the respective enclosures 12a, 12b, 12c, and 12d in asubstantially identical fashion as separator 40a is associated withenclosure 12a in the embodiment described in detail above. Theseparators 40a, 40b, 40c and 40d function in a substantially identicalmanner as separator 40a in the embodiment described in detail above. Theembodiment of FIG. 5 thus functions in the same manner as describedabove in connection with the embodiment of FIGS. 1-4 while enjoying theadded capacity and flexibility of the additional enclosures.

According to the embodiment of FIG. 6, two additional enclosures 12e and12f, which are substantially similar to enclosures 12a, 12b, 12c, and12d, are joined with enclosures 12a, 12b, 12c, and 12d. Enclosure 12e isdisposed adjacent enclosure 12b and opposite enclosure 12f, andenclosure 12f is disposed adjacent enclosure 12d. Common walls 130 and132, dividing enclosures 12b from 12e and 12d from 12f, respectively,are substantially identical to common walls 60 and 128. Similarly,common wall 134 dividing enclosure 12e from enclosure 12f, issubstantially identical to common walls 120 and 122. Separators 40a,40b, 40c, 40d, 40e, and 40f are associated with enclosures 12a, 12b,12c, 12d, 12e, and 12f, respectively. The embodiment of FIG. 6 functionsin the same manner as described above in connection with the embodimentof FIGS. 1-4 and the embodiment of FIG. 5 while enjoying the addedcapacity and flexibility of the additional enclosures.

Other modifications, changes and substitutions are intended in theforegoing disclosure and in some instances some features of theinvention will be employed without a corresponding use of otherfeatures. Accordingly, it is appropriate that the appended claims beconstrued broadly and in a manner consistent with the scope of theinvention.

What is claimed is:
 1. A fluidized bed combustion system comprising:anenclosure having a front wall, a rear wall, a first side wall and asecond side wall; a partition extending between said side walls anddividing said enclosure into a furnace section and a recycle section;means for introducing particulate material including fuel into saidfurnace section; means for introducing particulate material into saidrecycle section; means for passing particulate material from saidrecycle section to said furnace section; a grate disposed in a lowerportion of said enclosure for supporting said particulate material insaid furnace section and said recycle section; and means for introducingan oxygen-containing gas through said grate and into said furnacesection for fluidizing said particulate material in said furnacesection; and said first side wall having at least one aperture in alower portion of said side wall and registering with said recyclesection so that said particulate material in said recycle section maypass through said aperture.
 2. The system of claim 1 wherein:said firstside wall has at least one aperture in a lower portion of said side walland registering with said furnace section so that said particulatematerial in said furnace section may pass through said aperture; andsaid first side wall has at least one aperture in an upper portion ofsaid side wall and registering with said furnace section so that gasesin an upper portion of said furnace section may pass through saidaperture.
 3. The system of claim 2 wherein:said second side wall has atleast one aperture in a lower portion of said side wall registering withsaid furnace section so that said particulate material in said furnacesection may pass through said aperture; said second side wall has atleast one aperture in a lower portion of said side wall registering withsaid recycle section so that said particulate material in said recyclesection may pass through said aperture; and said second side wall havingat least one aperture in an upper portion of said side wall so thatgases in an upper portion of said furnace section may pass through saidaperture.
 4. The system of claim 2 wherein:said front wall has at leastone aperture in a lower portion of said front wall registering with saidfurnace section so that particulate material in said furnace section maypass through said aperture; and said front wall has at least oneaperture in an upper portion of said front wall so that gases in anupper portion of said furnace section may pass through said aperture. 5.The system of claim 2 wherein said first side wall is formed by aplurality of spaced cooling tubes connected by continuous fins to forman airtight structure, said apertures in said first side wall beingprovided by omitting said fins from between portions of said spacedtubes.
 6. The system of claim 2 further comprising:a second enclosurehaving a second furnace section and a second recycle section, saidsecond enclosure being disposed adjacent said enclosure and sharing saidfirst side wall so that said second furnace section is disposed adjacentsaid furnace section and said second recycle section is disposedadjacent said recycle section, said at least one aperture in said lowerportion of said side wall and registering with said furnace section alsoregistering with said second furnace section to permit particulatematerial to pass between said furnace section and said second furnacesection, said at least one aperture in a lower portion of said side walland registering with said recycle section also registering with saidsecond recycle section to permit particulate material to pass betweensaid recycle section and said second recycle section, and said at leastone aperture in an upper portion of said side wall and registering withsaid furnace section also registering with said second furnace sectionto permit gases to pass between said furnace section and said secondfurnace section.
 7. The system of claim 2 further comprising:a separatordisposed adjacent said enclosure for separating particulate materialfrom gases; means for passing particulate material from said upperportion of said furnace section to said separator; and wherein saidmeans for introducing particulate material into said recycle sectioncomprises a conduit for passing separated particulate material from saidseparator to said recycle section.
 8. The system of claim 7 wherein saidpartition has a first portion extending substantially vertically fromsaid grate and a second portion angling upwardly and rearwardly fromsaid first portion and extending to said rear wall.
 9. The system ofclaim 8 wherein said means for passing particulate material from saidrecycle section to said furnace section comprises:a second partitiondisposed between said side walls in said recycle section and extendingfrom said grate to said second portion of said partition to form achannel between said partition and said second partition, an upperportion of said second partition having an aperture for permittingparticulate material to pass from said recycle section to said channel,and a lower portion of said partition having an aperture for permittingparticulate material to pass from said channel to said furnace section.10. The system of claim 8 wherein said partition is formed by aplurality of spaced cooling tubes connected by continuous fins to forman airtight structure, said apertures in said partition being providedby omitting said fins from between portions of said spaced tubes.
 11. Afluidized bed combustion system comprising:a first enclosure having afirst furnace section; a first recycle section adjoining said firstfurnace section; means for forming a fluidized bed of particulatematerial including fuel in said first furnace section; means for forminga fluidized bed of particulate material in said first recycle section;means for passing particulate material from said first recycle sectionto said first furnace section; a second enclosure having a secondfurnace section; a second recycle section adjoining said second furnacesection; means for forming a fluidized bed of particulate materialincluding fuel in said second furnace section; means for forming afluidized bed of particulate material in said second recycle section;said first and second enclosure being disposed adjacently and sharing acommon wall which divides said first furnace section and said firstrecycle section from said second furnace section and said second recyclesection, said first furnace section being disposed adjacent said secondfurnace section and said first recycle section being disposed adjacentsaid second recycle section; said common wall having at least oneaperture extending through an upper portion of said common wall andregistering with said first and second furnace sections for permittinggases to pass between said first and second furnace sections to equalizepressure in said furnace sections; said common wall having at least oneaperture extending through a lower portion of said common wall andregistering with said first and second furnace sections for permittingparticulate material to pass between said first and second furnacesections; and said common wall having at least one aperture extendingthrough a lower portion of said common wall and registering with saidfirst and second recycle sections for permitting particulate material topass between said first and second recycle sections.
 12. The system ofclaim 11 wherein said common wall is formed by a plurality of spacedcooling tubes connected by continuous fins to form an airtightstructure, each of said apertures in said common wall being provided byomitting said fins from between portions of said spaced tubes.
 13. Thesystem of claim 11 further comprising:a first separator disposedadjacent said first enclosure for separating particulate material fromgases; means for passing particulate material and gases from said firstfurnace section to said first separator; means for passing separatedparticulate material from said first separator to said first recyclesection; a second separator disposed adjacent said second enclosure forseparating particulate material from gases; means for passingparticulate material and gases from said second furnace section to saidsecond separator; and means for passing separated particulate materialfrom said second separator to said second recycle section.
 14. Thesystem of claim 13 wherein said first and second separators are cycloneseparators.
 15. The system of claim 13 wherein said first recyclesection is located within said first enclosure and said second recyclesection is located within said second enclosure.
 16. The system of claim15 wherein:said first enclosure comprises a front wall, a rear wall, twoside walls, and a first grate for supporting said first and secondfluidized beds, said first recycle section being separated from saidfirst furnace section by a first partition, said first partition beingdisposed between said side walls of said first enclosure, a firstportion of said first partition extending substantially vertically fromsaid first grate, and a second portion of said first partition anglingupwardly and rearwardly from said first portion and extending to saidfirst rear wall; and said second enclosure comprises a front wall, arear wall, two side walls, and a second grate for supporting said thirdand fourth fluidized beds, said second recycle section being separatedfrom said second furnace section by a second partition, said secondpartition being disposed between said side walls of said secondenclosure, a first portion of said second partition extendingsubstantially vertically from said second grate, and a second portion ofsaid second partition angling upwardly and rearwardly from said firstportion and extending to said second rear wall.
 17. The system of claim16 wherein said first and second partitions are formed by a plurality ofspaced cooling tubes connected by continuous fins to form airtightstructures, and each of said apertures in said first and secondpartitions are provided by omitting said fins from between portions ofsaid spaced tubes.
 18. The system of claim 16 wherein:said means forpassing particulate material from said first recycle section to saidfirst furnace section comprises a third partition disposed between saidside walls of said first enclosure and extending in said first recyclesection from said first grate to said second portion of said firstpartition to form a first channel between said first partition and saidthird partition, an upper portion of said third partition having atleast one aperture for permitting particulate material to pass from saidfirst recycle section to said first channel, and a lower portion of saidfirst partition having at least one aperture for permitting particulatematerial to pass from said first channel to said first furnace section;and said means for passing particulate material from said second recyclesection to said second furnace section comprises a fourth partitiondisposed between said side walls of said second enclosure and extendingin said second recycle section from said second grate to said secondportion of said second partition to form a second channel between saidsecond partition and said fourth partition, an upper portion of saidfourth partition having at least one aperture for permitting particulatematerial to pass from said second recycle section to said secondchannel, and a lower portion of said second partition having at leastone aperture for permitting particulate material to pass from saidsecond channel to said second furnace section.
 19. The system of claim18 further comprising:a fifth partition disposed between said side wallsof said first enclosure, a lower portion of said fifth partition anglingupwardly into said first recycle section from said first rear wall tosaid second portion of said first partition, and an upper portion ofsaid fifth partition angling upwardly and rearwardly from said secondportion of said first partition of said first rear wall, said lowerportion of said fifth partition having at least one aperture; and asixth partition disposed between said side walls of said secondenclosure, a lower portion of said sixth partition angling upwardly intosaid second recycle section from said second rear wall to said secondportion of said second partition, and an upper portion of said sixthpartition angling upwardly and rearwardly from said second portion ofsaid second partition to said second rear wall, said lower portion ofsaid sixth partition having at least one aperture.
 20. The system ofclaim 19 further comprising:a first plurality of nozzles extendingthrough said second portion of said first partition and into said firstrecycle section below a point of intersection of said fifth partitionwith said second portion of said first partition; a second plurality ofnozzles extending through said second portion of said second partitionand into said second recycle section below a point of intersection ofsaid sixth partition with said second portion of said second partition;means for introducing an oxygen-containing gas through said first rearwall and into said first recycle section between points of intersectionof said upper and lower portions of said fifth partition with said firstrear wall so that said gas may create oxidizing conditions in said firstnozzles for combusting fine particulate material passing through saidfirst nozzles; and means for introducing an oxygen-containing gasthrough said second rear wall and into said second recycle sectionbetween points of intersection of said upper and lower portions of saidsixth partition with said second rear wall so that said gas may createoxidizing conditions in said second nozzles for combusting fineparticulate material passing through said second nozzles.
 21. The systemof claim 11 further comprising:a third enclosure having a third furnacesection; a third recycle section adjoining said third furnace section;means for forming a fluidized bed of particulate material including fuelin said third furnace section; means for forming a fluidized bed ofparticulate material in said third recycle section; means for passingparticulate material from said third recycle section to said thirdfurnace section; a fourth enclosure having a fourth furnace section; afourth recycle section adjoining said fourth furnace section; means forforming a fluidized bed of particulate material including fuel in saidfourth furnace section; means for forming a fluidized bed of particulatematerial in said fourth recycle section; said third enclosure beingdisposed opposite and sharing a second common wall with said firstenclosure; and said fourth enclosure being disposed opposite and sharinga third common wall with said second enclosure, and said fourthenclosure being disposed adjacent to and sharing a fourth common wallwith said third enclosure; said second common wall having at least oneaperture extending through a lower portion of said wall and registeringwith said first and third furnace sections permitting particulatematerial to pass between said first and third furnace sections, and saidsecond common wall having at least one aperture extending through anupper portion of said wall and registering with said first and thirdfurnace sections permitting gases to pass between said first and thirdfurnace sections; said third common wall having at least one apertureextending through a lower portion of said wall and registering with saidsecond and fourth furnace sections for permitting particulate materialto pass between said second and fourth furnace sections, and said thirdcommon wall having at least one aperture extending through an upperportion of said wall and registering with said second and fourth furnacesections for permitting gases to pass between said second and fourthfurnace sections; said fourth common wall having at least one apertureextending through a lower portion of said wall and registering with saidthird and fourth furnace sections for permitting particulate material topass between said third and fourth furnace sections, said fourth commonwall having at least one aperture in a lower portion of said wall andregistering with said third and fourth recycle sections for permittingparticulate material to pass between said third and fourth recyclesections, and said fourth common wall having at least one aperture in anupper portion of said wall and registering with said third and fourthfurnace sections for permitting gases to pass between said third andfourth furnace sections.
 22. The system of claim 21 wherein said firstrecycle section is disposed within said first enclosure, said secondrecycle section is disposed within said second enclosure, said thirdrecycle section is disposed within said third enclosure, and said fourthrecycle section is disposed within said fourth enclosure.
 23. The systemof claim 21 further comprising:a fifth enclosure having a fifth furnacesection; a fifth recycle section adjoining said fifth furnace section;means for forming a fluidizing bed of particulate material includingfuel in said fifth furnace section; means for forming a fluidized bed ofparticulate material in said fifth recycle section; means for passingparticulate material from said fifth recycle section to said fifthfurnace section; a sixth enclosure having a sixth furnace section; asixth recycle section adjoining said sixth furnace section; means forforming a fluidized bed of particulate material including fuel in saidsixth furnace section; means for forming a fluidized bed of particulatematerial in said sixth recycle section; said fifth enclosure beingdisposed adjacent said second enclosure on a side opposite said firstenclosure, and said fifth enclosure sharing a fifth common wall withsaid second enclosure; said sixth enclosure being disposed opposite saidfifth enclosure and adjacent said fourth enclosure, said sixth enclosuresharing a sixth common wall with said fifth enclosure and sharing aseventh common wall with said fourth enclosure; said fifth common wallhaving at least one aperture in a lower portion of said wall andregistering with said second and fifth furnace sections for permittingparticulate material to pass between said second and fifth furnacesections, said fifth common wall having at least one aperture in a lowerportion of said wall and registering with said second and fifth recyclesections for permitting particulate material to pass between said secondand fifth recycle sections, and said fifth common wall having at leastone aperture in an upper portion of said wall and registering with saidsecond and fifth furnace sections for permitting gases to pass betweensaid second and fifth furnace sections; said sixth common wall having atleast one aperture in a lower portion of said wall and registering withsaid fifth and sixth furnace sections for permitting particulatematerial to pass between said fifth and sixth furnace sections, and saidsixth common wall having at least one aperture in an upper portion ofsaid wall and registering with said fifth and sixth furnace sections forpermitting gases to pass between said fifth and sixth furnace sections;and said seventh common wall having at least one aperture in a lowerportion of said wall and registering with said fourth and sixth furnacesections for permitting particulate material to pass between said fourthand sixth furnace sections, said seventh common wall having at least oneaperture in a lower portion of said wall and registering with saidfourth and sixth recycle sections for permitting particulate material topass between said fourth and sixth recycle sections, and said seventhcommon wall having at least one aperture in an upper portion of saidwall and registering with said fourth and sixth furnace sections forpermitting gases to pass between said fourth and sixth furnace sections.24. The system of claim 23 wherein said first recycle section isdisposed within said first enclosure, said second recycle section isdisposed within said second enclosure, said third recycle section isdisposed within said third enclosure, said fourth recycle section isdisposed within said fourth enclosure, the fifth recycle section isdisposed within said fifth enclosure, and said sixth recycle section isdisposed within said sixth enclosure.
 25. A method of operating afluidized bed combustion system comprising the steps of:providing aplurality of enclosures, each of said enclosures having a furnacesection and a recycle section; introducing particulate materialincluding fuel into said furnace sections; introducing particulatematerial into said recycle sections; introducing an oxygen-containinggas into said furnace sections for fluidizing said particulate materialin said furnace sections and for promoting combustion of said fuel togenerate combustion gases in said furnace sections, and so that densebeds of particulate material having heights form in said furnacesections, and so that said fluidizing gas combines with said combustiongases to form flue gases which entrain a portion of said particulatematerial in said furnace sections; introducing an oxygen-containing gasinto said recycle sections for fluidizing said particulate material insaid recycle sections so that dense beds of particulate material havingheights form in said recycle sections; equalizing said heights of saiddense beds in said furnace sections; equalizing pressure in said furnacesections; and equalizing said heights of said dense beds in said recyclesections.
 26. The method of claim 25 wherein:said step of equalizingsaid heights of said dense beds in said furnace sections comprisespermitting particulate material to flow between said furnace sections;said step of equalizing said pressure in said furnace sections comprisespermitting said flue gases to flow between said furnace sections; andsaid step of equalizing said heights of said dense beds in said recyclesections comprises permitting particulate material to flow between saidfurnace sections.
 27. The method of claim 26 further comprising:passinga portion of said flue gases and said entrained particulate materialfrom said furnace sections; separating said passed, entrainedparticulate material from said flue gases; and wherein said step ofintroducing particulate material into said recycle sections comprisespassing said separated particulate material to said recycle sections.28. The method of claim 27 further comprising removing heat from saidseparated particulate material in said recycle sections.
 29. The methodof claim 27 wherein:said particulate material in said recycle sectionsincludes fine fuel particles; and further comprising controlling saidfluidizing gas introduced into said recycle sections to entrain saidfine fuel particles said dense beds in said recycle sections; passing aportion of said entrained fine fuel particles from said recycle sectionsto said furnace sections; and introducing a secondary oxygen-containinggas into said recycle sections above said dense beds so that saidsecondary gas creates oxidizing conditions for combusting said entrainedfine fuel particles as said entrained fine fuel particles pass from saidrecycle sections to said furnace sections.